The Christchurch terror attacks sparked an unmatched surge in white supremacist activity online, according to a US-based intelligence firm.

The director of the SITE Intelligence Group, Rita Katz, told The Sydney Morning Herald the online rallying behind the March 15 massacres had been unprecedented for right-wing attacks.

A total of 50 people were killed when a lone gunman opened fire on two Christchurch mosques.

It was the deadliest shootings in New Zealand's modern history.

"Attacks always spark reactions from different extremist communities, but when it comes to the far right, there was never anything like the response to the Christchurch attack," Katz told The Sun-Herald.